Various heat welding devices are known for fusing together weldable components and work-pieces. One known heat welding device incorporates a hot plate which by thermal contact melts part of the component and/or the work-piece to a tacky state, after which the component and workpiece are brought into physical contact and then cooled, fusing together to produce a finished article. Frequently, hot plate welders are used in the production of thermoplastic automobile parts, joining components, such as filler necks, brackets, vent nipples, blow pin plugs and the like, to fuel tanks, anti-freeze and windshield washer fluid reservoirs and other work-pieces.
Conventional hot plate welders include a vertically movable carrier or holding fixture and a heating unit. The holding fixture holds a component to be mounted in a fixed orientation moving it into contact with an upper surface of a work-piece. The heating unit heats and partially melts one or both of the contact surface of the component, namely the part of the component which is to be brought into sealing contact with the work-piece, and a portion of the upper surface of the work-piece where the component is to be attached. In use, the heating unit is activated to melt the respective portions of the component and work-piece. After the melting of the component and work-piece is complete, the holding fixture is moved lowering the component held thereby along a vertical axis aligned with the melted portion of the work-piece, until the melted contact surface to the component is brought into sealing contact against the melted portion of the work-piece.
In one known hot plate welder, the heating unit is movable both vertically along an axis into and from contact with the work-piece, as well as in a horizontal direction. The heating unit is provided with aligned upper and lower melting surfaces, the upper surface for engaging and melting the contact surface of the component, and the lower surface for engaging and melting the portion of the work-piece. In operation, the holding fixture with the component coupled thereto, the heating unit and the portion of the work-piece to which the component is to be attached are all axially aligned. The holding fixture is lowered to move the component against the upper melting surface of the heating unit, melting the contact surface of the component. The downward force of the component on the upper melting surface in turn biases the heating unit downwardly, moving the lower melting surface to contact and melt the work-piece. Once the component and the work-piece are sufficiently melted, the holding fixture and heating unit are both partially raised, and the heating unit is moved horizontally to a position clear of the holding fixture. The holding fixture is then again lowered to move the still melted contact surface of the component against the melted portion of the work-piece, fusing the component thereto.
A difficulty with conventional hot plate welders is that the requirement that both the work-piece melting surface and the component carrying or holding fixture move along an axis which is 90.degree. to the surface of the work-piece. This necessitates precise and time consuming calibration of the welder to ensure that the surface of each separate work-piece is in the exact 90.degree. orientation. With conventional hot plate welders, it is therefore not possible to simply change production lines to produce a different article, a difficulty which is particularly compounded where a number of different components are to be secured to a number of different locations on each single work-piece.
Another major difficulty with conventional hot plate welders is that they are most frequently used to secure components to work-pieces having uneven or non-uniform surfaces, particularly in the case of blow molded thermoplastic work-pieces. Blow molded work-pieces are frequently formed without uniform wall thickness and shape as a result of uneven shrinkage as the thermoplastic cools. The result is therefore that the surface of the individual work-pieces will vary in orientation, and after initial calibration of the welder, are unlikely to be precisely 90.degree. to the axis of movement of each of the work-piece heating unit and the component carrier or holding fixture. As such when the heating unit engages the workpiece, there will be uneven contact, with the result that a portion of the work-piece may remain incompletely, or even entirely unmelted. When the melted component is moved against the work-piece, there is again uneven contact between the contact surface of the component and the unevenly melted portion of the work-piece, with the result that an incomplete weld may result which may later lead to the premature failure of the produced article.
A further disadvantage of conventional hot plate welders is that except in severe cases where an incomplete weld is visually apparent, there is no way for the operator to determine whether uneven welding has occurred, and if so, whether weld is outside acceptable tolerances so as to likely lead to failure.
Consideration has been given to providing an apparatus in which the work-piece heating unit, which contacts and melts the work-piece, is movable to a parallel orientation with the work-piece surface to be melted. The inventor has, however, appreciated a difficulty with such a structure in that while the work-piece may be evenly melted, the component remains fixed within the holding fixture. The result may therefore be that even more uneven sealing may occur as a result of the fixedly held component engaging an angularly melted surface.